Piezoelectric signal crystals having a Perovskite-type crystal structure have much higher dielectric constant (K3T) and piezoelectric constants (d33 and k33) than existing piezoelectric polycrystals, and are used in high performance parts, such as a piezoelectric actuator, a piezoelectric transducer, a piezoelectric sensor, and the like, so that they are expected to be adapted to substrate materials of various thin-film devices.
As the piezoelectric single crystals having a Perovskite-type crystal structure, which have been developed, there are PMN—PT (Pb(Mg1/3Nb2/3)O3—PbTiO3), PZN—PT (Pb(Zn1/3Nb2/3)O3—PbTiO3), PInN—PT (Pb(In1/2Nb1/2)O3—PbTiO3), PYbN—PT (Pb(Yb1/2Nb1/2)O3—PbTiO3), PSN—PT (Pb(Sc1/2Nb1/2)O3—PbTiO3), PMN—PInN—PT, PMN—PYbN—PT, BiScO3—PbTiO3 (BS—PT), and others. These single crystals exhibited congruent melting upon melting, and were generally prepared by a flux method, a Bridgman method, and others, which are the existing single crystal growth methods.
The piezoelectric single crystals, which had been developed, such as PMN—PT, PZN—PT, and the like, have an advantage of showing high dielectric and piezoelectric characteristics at room temperature (K3T>4,000), d33>1,400 pC/N and k33>0.85), but also have defects of having low phase transition temperatures (Tc and TRT), low coercive field (Ec), brittleness, and the like, so that conditions such as usable temperature range, usable voltage, or the like of the piezoelectric single crystals, and fabrication conditions of piezoelectric single crystal application parts come to be greatly limited. Generally, it has been known that Perovskite-type piezoelectric single crystals have the highest dielectric and piezoelectric characteristics at a phase boundary between a rhombohedral phase and a tetragonal phase, i.e., around a morphotropic phase boundary (MPB) composition. Tetragonal piezoelectric single crystals have been known to be usable in some of specified crystalline orientations having excellent piezoelectric or electrooptical properties.
However, since the Perovskite-type piezoelectric single crystals in general show the best excellent dielectric and piezoelectric characteristics when they are rhombohedral phases, rhombohedral piezoelectric single crystals are widely applied. However, since the rhombohedral piezoelectric single crystals are stable only below a phase transition temperature (TRT) between a rhombohedral phase and a tetragonal phase, they are usable only below TRT that is the highest temperature at which the rhombohedral phase can be stable. Thus, when the phase transition temperature (TRT) is low, the usable temperature of the rhombohedral piezoelectric single crystal comes to low, and the fabrication and use temperatures of the piezoelectric single crystal application parts are restricted below TRT. In addition, when the phase transition temperatures (Tc and TRT) and coercive field (Ec) are low, under mechanical machining, strains, heat generation, and driving voltages, the piezoelectric single crystals are easily depoled, and excellent dielectric and piezoelectric characteristics thereof are lost.
Accordingly, piezoelectric single crystals having low phase transition temperatures (Tc and TRT) and coercive field (Ec) are restricted in conditions on fabrication of application parts thereof, usable temperatures, driving voltages, and the like. In the case of PMN—PT single crystal, in general, it is in the state of Tc<150° C., TRT<80° C., and Ec<2.5 kV/cm. In the case of PZT—PT single crystal, in general, it is in the state of Tc<170° C., TRT<100° C., and Ec<3.5 kV/cm.
Moreover, dielectric and piezoelectric application parts fabricated by using such piezoelectric single crystals are also restricted in their conditions on fabrication, usable temperature ranges, usable voltages, or the like, so that it is difficult to develop and realize the piezoelectric single crystal application parts.
In order to overcome disadvantages of the piezoelectric single crystals, there has been developed a single crystal having a new composition, such as PInN—PT, PSN—PT, BS—PT, and the like, and there also has been studied combination-type compositions of the single crystal, such as PMN—PInN—PT, PMN—BS—PT, and the like. In the case of these single crystals, however, dielectric constant, piezoelectric constant, phase transition temperatures, coercive field, and mechanical characteristics thereof cannot be totally improved. Further, the piezoelectric single crystals essentially composed of Sc, In, and the like, which are expensive, are under a difficulty in putting to practical use due to their high production cost.
The reason why the presently developed Perovskite-type piezoelectric single crystals have low phase transition temperatures may be divided into following three cases. First, as shown in Table 1, the reason is because a phase transition temperature of a relaxer (such as PMN, PZN, or the like), which may be an essential element, together with PT, is low. Table 1 shows phase transition temperatures (Curie Temperatures, Tc) between tetragonal phases and cubic phases of Perovskite piezoelectric ceramic polycrystals (Ref.: Park et al., “Characteristics of Relaxor-Based Piezoelectric Single Crystals for Ultrasonic Transducers,” IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol. 44, no. 5, 1997, pp. 1140-1147). Since the Curie temperature of the piezoelectric single crystal is similar to that of the polycrystal having the same composition, it can be estimated the Curie temperature of the piezoelectric single crystal from that of the polycrystal. Second, the reason is because MPB forming a boundary between a rhombohedral phase and a tetragonal phase does not become perpendicular to a temperature axis, and is gently inclined. Thus, since the Curie temperature (Tc) is essentially reduced in order to raise the phase transition temperatures (TRT), it is difficult to raise the Tc, together with the TRT. Third, the reason is because, also in the case where the relaxer (PYbN, PInN, BiScO3, or the like) having relatively high Tc is mixed with PMN—PT and the like, there is a problem in that the phase transition temperature does not simply increase in proportion to the composition, or dielectric and piezoelectric characteristics are degraded.
TABLE 1Binary systems (Relaxor-PT)PT contentTc [° C.] ofTc [° C.] of(Tc of PbTiO3) = 490° C.)on MPBMPBend component(1−x)Pb(Zn1/3Nb2/3)O3—xPbTiO3 (PZN-PT)x ≈ 0.09~180140(1−x)Pb(Mg1/3Nb2/3)O3—xPbTiO3 (PMN-PT)x ≈ 0.33~150−10(1−x)Pb(Mg1/3Ta2/3)O3—xPbTiO3 (PMT-PT)x ≈ 0.38~80−98(1−x)Pb(Ni1/3Nb2/3)O3—xPbTiO3 (PNN-PT)x ≈ 0.40~170−120(1−x)Pb(Co1/3Nb2/3)O3—xPbTiO3 (PCoN-PT)x ≈ 0.38~250−98(1−x)Pb(Sc1/2Ta1/2)O3—xPbTiO3 (PST-PT)x ≈ 0.45~20526(1−x)Pb(Sc1/2Nb1/2)O3—xPbTiO3 (PSN-PT)x ≈ 0.43~25090(1−x)Pb(Fe1/2Nb1/2)O3—xPbTiO3 (PFN-PT)x ≈ 0.07~140110(1−x)Pb(Yb1/2Nb1/2)O3—xPbTiO3 (PYbN-PT)x ≈ 0.50~360280(1−x)Pb(In1/2Nb1/2)O3—xPbTiO3 (PIN-PT)x ≈ 0.37~32090(1−x)Pb(Mg1/2W1/2)O3—xPbTiO3 (PMW-PT)x ≈ 0.55~6039(1−x)Pb(Co1/2W1/2)O3—xPbTiO3 (PCoW-PT)x ≈ 0.45~31032(1−x)PbZrO3—xPbTiO3 (PZT)x ≈ 0.48~360230
Relaxer-PT based single crystals of Table 1 are prepared by flux method or Bridgman method, which is existing single crystal growth method generally using a melting process. However, such single crystals were not yet put to practical use because of difficulty in producing a large single crystal having uniform composition, high production cost, and difficulty in mass production.
Generally, the piezoelectric single crystals have lower mechanical strength and fracture toughness relative to the piezoelectric polycrystalline ceramics, to thereby have a defect of being easily broken by even low mechanical impact. Brittleness of the piezoelectric single crystals causes easy breakage of the piezoelectric single crystals during fabrication and use of the piezoelectric single crystal application parts, which greatly restricts the use of piezoelectric single crystals. Accordingly, there is a need to improve mechanical properties of the piezoelectric single crystals, together with dielectric and piezoelectric characteristics thereof, in order for commercialization of the piezoelectric single crystals.
Disclosure
Technical Problem
The present invention provides piezoelectric single crystals having a Perovskite-type crystal structure, in particular, which have a high dielectric constant (K3T≧4,000 to 8,000), high piezoelectric constants (d33≧1,400 pC/N to 2,500 pC/N and k33≧0.85 to 0.95), high phase transition temperatures (Tc≧180° C. to 400° C. and TRT≧100° C. to 250° C.), a high coercive field (Ec≧5 kV/cm to 15 kV/cm) and improved mechanical properties.
Unlike the existing Perovskite-type piezoelectric single crystals containing expensive elements such as Sc and In as the major component, the invention introduces a novel composition of Perovskite-type piezoelectric single crystals, which never or rarely contain expensive elements but have excellent characteristics, to lower single crystal production costs, thereby enabling commercialization of the piezoelectric single crystals.
Furthermore, the invention provides dielectric and piezoelectric application parts including Perovskite-type piezoelectric single crystals having all of high dielectric constant (K3T), high piezoelectric constants (d33 and k33), high phase transition temperatures (Tc and TRT) and high coercive field (Ec) so that the dielectric and piezoelectric application parts using the piezoelectric single crystals of excellent characteristics can be produced and used in high temperature ranges. The invention also provides a method of growing single crystals which is different from existing single crystal growth methods such as flux and Bridgman methods, and can employ a solid-state crystal growth method in order to mass produce single crystals at low cost according to a general heat-treatment process without having to use specific apparatuses.
Furthermore, the invention provides piezoelectric single crystals having a Perovskite structure which are highly resistant against mechanical impact but have good machinability. Accordingly, the application parts can be fabricated easily by using the piezoelectric single crystals so that the fracture or deterioration of the application parts in use can be prevented.
Technical Solution
To obtain the foregoing objects, a piezoelectric single crystal having a Perovskite-type crystal structure ([A][B]O3) of the invention contains Zr.
The piezoelectric single crystal has a composition expressed by formula 1 below:[A][(MN)(1-x-y)TixZry]O3   [Formula 1]wherein A is at least one selected from the group consisting of Pb, Sr, Ba and Bi, M is at least one selected from the group consisting of Ce, Co, Fe, In, Mg, Mn, Ni, Sc, Yb and Zn, N is one selected from the group consisting of Nb, Sb, Ta and W, and x and y satisfy, by mole fraction, following relationships:0.05≦x≦0.58, and0.05≦y≦0.62.
In formula 1 above, A is preferably Pb. That is, the composition is preferably expressed by formula 2:[Pb][MN)(1-x-y)TixZry]O3   [Formula 2]
In formula 1 above, N is preferably Nb. That is, the composition is expressed by formula 3:[A][((M)(Nb))(1-x-y)TixZry]O3   [Formula 3]
The piezoelectric single crystal having the composition of formula 1 preferably has a composition expressed by formula 4:[Pb(1-a-b)SraBab][((Mg,Zn)1/3Nb2/3)(1-x-y)TixZry]O3   [Formula 4]where a and b satisfy following relationships: by mole fraction, 0.0≦a≦0.1, and, 0.0≦b≦0.6.
The piezoelectric single crystal having the composition of formula 1 preferably has a composition expressed by formula 5:[Pb][((Mg(1-a)Zna)1/3Nb2/3)(1-x-y)TixZry]O3   [Formula 5]where x and a satisfy following relationships: by mole fraction, 0.20≦x≦0.58, and 0.0≦a≦0.5.
The piezoelectric single crystal having the composition of formula 1 preferably has a composition expressed by formula 6:[Pb][(Mg1/3Nb2/3)(1-x-y)TixZry]O3   [Formula 6]where x satisfies a following relationship: by mole fraction, 0.25≦x≦0.58.
The piezoelectric single crystal having the composition of formula 1 preferably has a composition expressed by formula 7:[BaxBi(1-x)][Fe(1-x)Ti(x-y)Zry]O3   [Formula 7]where x and y satisfy following relationships: by mole fraction, 0.65≦x≦1.00 and 0.05≦y≦0.15.
Furthermore, the piezoelectric single crystal preferably has a composition wherein P is added to any one of the compositions of formulas 1 to 7. Particularly, the piezoelectric single crystal preferably has a composition according to any one of formulas 8 to 14 below. P exists in the form of second phase in the piezoelectric single crystal, and preferably, a second phase of one selected from the group consisting of metals, oxides and pores. Furthermore, P is at least one selected from the group consisting of metals [Au (Gold), Ag (Silver), Ir (Iridium), Pt (Platinum), Pd (Palladium), Rh (Rhodium)], oxides [MgO and ZrO2] and pores. P added is preferably in the range from 0.1% to 20% by volume fraction with respect to the whole composition.[A][(MN)(1-x-y)TixZry]O3+cP   [Formula 8]
In formula 8, A is at least one selected from the group consisting of Pb, Sr, Ba and Bi, M is at least one selected from the group consisting of Ce, Co, Fe, In, Mg, Mn, Ni, Sc, Yb and Zn, and N is one selected from the group consisting of Nb, Sb, Ta and W. In addition, P exists in the form of second phase in the piezoelectric single crystal, and preferably, a second phase of one selected from the group consisting of metals (Au, Ag, Ir, Pt, Pd and Rh), oxides (MgO and ZrO2) and pores. Furthermore, c satisfies, by volume fraction, the following relationship: 0.001≦c≦0.20, and x and y satisfy, by mole fraction, following relationships:0.05≦x≦0.58.0.05≦y≦0.62[Pb][(MN)(1-x-y)TixZry]O3+cP   [Formula 9]
In formula 9 above, M, N, x, y, P and c are equal as defined in formula 8 above.[A][((M)(Nb))(1-x-y)TixZry]O3+cP   [Formula 10]
In formula 10 above, A, M, x, y, P and c are equal as defined in formula 8 above.[Pb(1-a-b)SraBab][((Mg,Zn)1/3Nb2/3)(1-x-y)TixZry]O3+cP   [Formula 11]
In formula 11 above, x, y, P and c are equal as defined in formula 8 above, a satisfies, by mole fraction, a relationship: 0.0≦a≦0.1, and b satisfies, by mole fraction, a relationship: 0.0≦b≦0.6.[Pb][((Mg(1-a)Zna)1/3Nb2/3)(1-x-y)TixZry]O3+cP   [Formula 12]
In formula 12 above, y, P and c are equal as defined in formula 8 above, x satisfies, by mole fraction, a relationship: 0.20≦x≦0.58, and a satisfies, by mole fraction, a relationship: 0.0≦a≦0.5.[Pb][(Mg1/3Nb2/3)(1-x-y)TixZry]O3+cP   [Formula 13]
In formula 13 above, y, P and c are equal as defined in formula 8 above, and x satisfies, by mole fraction, a relationship: 0.25≦x≦0.58.[BaxBi(1-x)][Fe(1-x)Ti(x-y)Zry]O3+cP   [Formula 14]
In formula 14 above, P and c are equal as defined in formula 8 above, x and y satisfy, by mole fraction, relationships: 0.65≦x≦1.00 and 0.05≦y≦0.15.
The piezoelectric single crystals having any one of the compositions according to formulas 1 to 14 show all of the following properties: dielectric constant of K3T>4,000; piezoelectric constant of d33≧1,400 pC/N and k33≧0.85; phase transition temperatures of Tc≧180° C. and TRT≧100° C. and coercive field of Ec≧5 kV/cm.
The invention also relates to dielectric and piezoelectric application parts which include piezoelectric single crystals having a Perovskite crystal structure expressed by any one of formulas above and thus show all of the following properties: dielectric constant of K3T≧4,000; piezoelectric constant of d33≧1,400 pC/N and k33≧0.85; phase transition temperatures of Tc≧180° C. and TRT≧100° C. and coercive field of Ec≧5 kV/cm.
The invention also relates to dielectric and piezoelectric application parts which include lead-free piezoelectric single crystals having a lead-free Perovskite crystal structure expressed by any one of formulas above and thus are environmental friendly free from toxic lead (Pb).
Furthermore, the piezoelectric single crystals having the composition according to any one of formulas 8 to 14 contain at least one reinforcing second phase (P) selected from the group consisting of metals (Au, Ag, Ir, Pt, Pd and Rh), oxides (MgO and ZrO2) and pores. With improved mechanical properties, the piezoelectric single crystals provide large resistance against mechanical impact but good machinability. In particular, in case of the second phase of metals (Au, Ag, Ir, Pt, Pd and Rh), dielectric and piezoelectric characteristics are also improved.
Furthermore, the piezoelectric single crystals having the composition according to any one of formulas 8 to 14 contain at least one reinforcing second phase (P) selected from the group consisting of metals (Au, Ag, Ir, Pt, Pd and Rh), oxides (MgO and ZrO2) and pores, in which the reinforcing second phase (P) is uniformly distributed in form of particles or regularly distributed in a specific pattern in the piezoelectric single crystal. According to the distribution type of the reinforcing second phase, dielectric, piezoelectric and mechanical properties of the piezoelectric single crystals are improved.
The invention also provides a method of producing piezoelectric single crystals having a composition expressed by any one of formulas above. The method of producing piezoelectric single crystals includes: (a) controlling average size of matrix grains of a polycrystal having said composition to reduce number density (ND) of abnormal grains; and (b) heat-treating said polycrystal with the number density of abnormal grains reduced through the step (a) to grow the abnormal grains.
Furthermore, the method of producing piezoelectric single crystals includes: (a) controlling the composition, heat-treatment temperature and heat-treatment atmosphere to promote abnormal grain growth in a polycrystal and controlling average size of matrix grains of the polycrystal to reduce number density (ND) of abnormal grains; and (b) heat-treating said polycrystal with the number density of abnormal grains reduced through the step (a) to grow the abnormal grains. In this fashion, it is possible to continuously grow only the reduced number of abnormal grains free from interference from surrounding abnormal grains or continuously grow the single crystal seed into the polycrystal to produce single crystals having a size of 50 mm or more.
In the method of producing piezoelectric single crystals of the invention as described above, after the single crystal seed is attached to the polycrystalline material before the heat-treatment, the heat-treatment is performed in conditions that the abnormal grain growth is induced in a joint but suppressed inside the polycrystal so that the single crystal seed is continuously grown in the polycrystal.
In the method of producing piezoelectric single crystals of the invention as described above, the average size of the matrix grains of the polycrystal is controlled according to a following relationship: 0.5 Rc≦R≦2 Rc, where R is the average size of the matrix grains, and Rc is the critical size of the matrix grains at which an abnormal grain growth starts to occur and the number density of abnormal grains becomes zero.
Furthermore, in the method of producing piezoelectric single crystals of the invention as described above, in case of attempting to create and grow merely the reduced number of abnormal grains, the average size of the matrix grains of the polycrystal is controlled according to a following relationship: 0.5 Rc≦R≦Rc, where R is the average size of the matrix grains, and Rc is the critical size of the matrix grains at which an abnormal grain growth starts to occur and the number density of abnormal grains becomes zero.
Advantageous Effects
The piezoelectric single crystals and piezoelectric single crystal application parts of the invention have all of high dielectric constant (K3T), high piezoelectric constants (d33 and k33), high phase transition temperatures (Tc and TRT), high coercive field (Ec) and improved mechanical properties and thus can be used in high temperature ranges and high voltage conditions.
Furthermore, the piezoelectric single crystals are produced by the solid-state single crystal growth (SSCG) method adequate for mass production of single crystals and the single crystal composition is developed not to contain expensive raw materials so that the piezoelectric single crystals can be easily commercialized. The piezoelectric and dielectric application parts using the piezoelectric single crystals of excellent properties according to the invention can be produced and used in the wide temperature range.